External Ze

[Eckaslike]

Have just completed an extension & fixed a new 10W dual RCCD Consumer unit for new circuits & existing. Started testing & discovered that although the test buttons on both RCCD work they wont trip when carrying out a RCCD test. I tested the external Ze & obtained a reading of 750 ohms, underground service with an earth clamp fitted.
I checked the earth clamp on the incoming cable which is the old type with lead sheath with steel protection and the connection is good. So should I install an Earth rod
connected to the earthblock & write this on the Electrical Certificate. Any advise would be apprecated, thanks.

 

[telectrix]

first, if you have a TN-S with that Ze, call out the DNO. it's their problem. as for the RCDs not tripping, that's got a few posssibilities maybe unrelated to the Ze.

 

[polo1]

Hence, why you should always check Ze/Zs before starting work!

 

[mechelec]

I guess you have left the installation energised ? As stated previously call DNO immediately. Don't forget you are the last person to do any work on this installation and you have left it with rcd protection that will not operate and an ineffective earth. Good luck if for the future if you are going to work like this.

 

[tony mc]

As for you RCD not tripping. On testing on x 1 with a Ra of 750 the RCD should still trip as it is well below 1666 ohms limit re touch voltage so you need to start scratching your head. A bit more information would help as to what you meter is displaying on the screen after the test and what make of meter. I take it a Temp ROD has been installed until the earth is sorted out ?

 

[Marvo]

I also think you've got two separate problems if your Ze measurement is accurate and I'd class both the high impedance earth and non-operation of the RCD as urgent.

 

[GLENNSPARK]

Have just completed an extension & fixed a new 10W dual RCCD Consumer unit for new circuits & existing. Started testing & discovered that although the test buttons on both RCCD work they wont trip when carrying out a RCCD test. I tested the external Ze & obtained a reading of 750 ohms, underground service with an earth clamp fitted.
I checked the earth clamp on the incoming cable which is the old type with lead sheath with steel protection and the connection is good. So should I install an Earth rod
connected to the earthblock & write this on the Electrical Certificate. Any advise would be apprecated, thanks.

so...
here we are again...
board changes without any pre-testing having been carried out...

how many more times does it have to be...

You IR test finals and between neutrals before any board change

You get a Ze before any board change

it aint hard is it....

 

[Amp David]

It is when you probably aren't properly qualified:bigcry:

- - - Updated - - -

Just chuck it in, hope for the best, then if it starts going ----, come on Electricians Forum for help.

 

[telectrix]

As for you RCD not tripping. On testing on x 1 with a Ra of 750 the RCD should still trip as it is well below 1666 ohms limit re touch voltage so you need to start scratching your head. A bit more information would help as to what you meter is displaying on the screen after the test and what make of meter. I take it a Temp ROD has been installed until the earth is sorted out ?

tony, the 1666 figure is to ensure that the RCD will trip as fault protection. it's function for providing additional protection is not dependant on the value of the Ze, if any.

 

[tony mc]

I agree Tel but if the Ra Value is above 1666 the meter will not allow the test as the touch voltage on the system will rise above 50 v so the meter will stop the test if that makes sense ! That is when doing a 1 x test with a 30 mA RCD. With the 5 x test the Ra will have to be lower than 333 ohms or the meter will again stop the test if set to 50 v touch which it should be ! Tony

 

[telectrix]

tha, i think is because the meter puts a fault to earth when doing the test. obviously, if the earth terminal the meter connects to is not earthed, the RCD will not see the fault. if the meter actually put a L-N imbalance, then it would work.

 

[tony mc]

IMO but I might be wrong the meter needs a earth connection to start the test even with a 3 lead test if Line & Neutral were connected without earth the meter would not allow the test. As for a imbalance with the Line & Neutral I suppose you could connect the earth lead to the neutral causing a imbalance but I would not be doing that. I see the op has not been back on with the questions asked so we will never find out. He states the test button works but I wouldn't be counting on that IMO ! Marvo & Glenn have summed it up !

 

[oldtimer]

This is why I no longer get involved in these types of posts yes I know people need help but I can no longer coach someone with 15 posts on a job like this. Plus the premise of chuck it in cos you can do no harm frankly worries me.

 

[Octopus]

So in one of the OP's earlier threads "I am a time served sparks with 50 years experiance but not Part P"

bxllxcks is all I can say.